Natural gas fuels are relatively environmentally friendly for use in vehicles, and hence there is support by environmental groups and governments for the use of natural gas fuels in vehicle applications. Natural gas based fuels are commonly found in three forms: Compressed Natural Gas (CNG), Liquefied Natural Gas (LNG) and a derivative of natural gas called Liquefied Petroleum Gas (LPG).
Natural gas fuelled vehicles have impressive environmental credentials as they generally emit very low levels of SO2 (sulphur dioxide), soot and other particulate matter. Compared to gasoline and diesel powered vehicles, CO2 (carbon dioxide) emissions of natural gas fuelled vehicles are often low due to a more favourable carbon-hydrogen ratio found in natural gas. Natural gas vehicles come in a variety of forms, from small cars to buses and increasingly to trucks in a variety of sizes. Natural gas fuels also provide engines with a longer service life and lower maintenance costs. Further, CNG is the least expensive alternative fuel when comparing equal amounts of fuel energy. Still further, natural gas fuels can be combined with other fuels, such as diesel, to provide similar benefits mentioned above.
A key factor limiting the use of natural gas in vehicles is the storage of the natural gas fuel. In the case of CNG and LNG, the fuel tanks are generally expensive, large and cumbersome relative to tanks required for conventional liquid fuels having equivalent energy content. In addition, the relative lack of wide availability of CNG and LNG refueling facilities, and the cost of LNG, add further limitations on the use of natural gas as a motor vehicle fuel. Further, in the case of LNG, the cost and complexity of producing LNG and issues associated with storing a cryogenic liquid on a vehicle further limits the widespread adoption of this fuel.
Some of the above issues are mitigated when using LPG and this fuel is widely used in high mileage motor cars such as taxis. However, cost versus benefit comparisons are often not favourable in the case of private motor cars. Issues associated with the size and shape of the fuel tank, the cost variability of LPG and the sometimes limited supply mean that LPG also has significant disadvantages that limit its widespread adoption. In summary, unless there is massive investment in a network of LNG plants around major transport hubs, CNG is the only feasible form of natural gas that is likely to be widely utilised in the near future.
Further; although LNG has had some success as a liquid fuel replacement in some regions of the world, the lack of availability of LNG and its high cost means that in many regions of the world it is not feasible to use LNG. In the case of CNG, it also has had some success as a liquid fuel replacement but almost exclusively in spark ignition engines utilising the low pressure carburetted port injection induction technology. This application is popular in government bus fleets around the world where the cleaner burning natural fuel is used in a spark ignition engine fitted in place of a conventional diesel engine. The application is usually a limited range fleet and includes a buffer CNG fill strategy with overnight refueling of the fleet.
However, the circumstances for broad implementation of CNG in large vehicles are limited by this buffer fill strategy, which essentially delivers gas at only the capacity of the compressor with any gas storage acting as a buffer to minimise compressor on/off cycling. Thus CNG has been seen as having limitations due to the size of incoming gas connections and electrical power requirements to meet intermittent and peak demands at refueling stations.
For example, a typical requirement for refueling a CNG vehicle is 10 diesel gallons equivalent per minute. If 4 vehicles were to be refueled simultaneously, on a site with 4 dispensers, this would require up to 2000 kW of compression and a correspondingly large gas interconnection, if using typical US industry CNG industrial gas supply connection pressures.
U.S. Pat. No. 4,805,674 to Knowlton discloses a “fast-fill” natural gas storage and retrieval system that overcomes some of the above described problems regarding the need for significant energy to compress natural gas from the relatively low pressure of utility supply lines to the on-vehicle storage pressures of around 3600 psig. Knowlton uses a natural gas displacing liquid to effectively vary the volume of a primary CNG storage vessel.
However, the disclosure of Knowlton presents several problems regarding gas loss and gas contamination. For example, if the displacing liquid is an aqueous liquid, the CNG can become contaminated with water, which requires expensive gas drying processes when the gas is expelled for use. Further, alternative displacing liquids can become contaminated by the CNG dissolving into the liquid. The dissolved CNG then can be lost when the displacing liquid is removed from the CNG storage vessel to a low-pressure liquid storage tank.
Ionic liquids, i.e., a salt in liquid state with low vapour pressure, have been trialed with CNG displacement in micro scale compressors, but these solutions are expensive, often flammable, and have high environmental toxicity—and thus do not scale to large installations.
Further various solutions of hydrocarbon type oils have been trialed with poor results, as these solutions take up a substantial quantity of gas in solution, which presents a problem with gas recovery and otherwise loss when the solution is returned to a low-pressure liquid storage tank.
Also, gas and liquid isolation inside pressure vessels has been attempted using physical bladders or mechanical pistons; however problems with cost, complexity, scaling, fabrication and maintenance have made these potential solutions problematic.
Therefore, there is a need for an improved system and method for refueling compressed gas pressure vessels.